The only serious problem here are the sectors, so just get rid of them by editing scene.4ds in HW (don't delete them! just put some basic dummy object instead of them, to keep the original hierarchy). Lightmaps can be renamed and exported/imported quite easily. You'll probly have to make all the collisions by yourself, but that won't take much time, it's all about roads and fences.
Good luck!

It's not impossible, it's not even hard. Just very time consuming. I'm also trying to make my own version. So far I have completely merged scene.4ds, made most collisions, added DeadFall Pass and nearly finished 2nd PS2 location. I'll have to edit red circled area and I can start with merging traffic. Scene2.bin seems to be no problem thanks to dnc's.

Thanks Yeah, it looks quite empty and vast. I've finished 2nd PS2 location, edited tunel sector a bit, added its lights and definition. Imported all sounds and music, wrote my first script for ambient sound transition between town and countryside It checks what direction you are passing detector by, what ambient is currently playing and acts accordingly.

I gave a go at traffic merging and it's going fine but fixing points will take some time. Hell, if not for macros I wouldn't do it I will also make countryside traffic less dense than the one in the city.